1. Field of the Invention
The present invention relates to a toner which is used to form an image out of a toner image contact-heated and fixed on paper by a heat roller in a dry type electrostatic copying process, a method for manufacturing the toner, and an image forming apparatus using the toner.
2. Description of the Related Art
Conventionally, a contact-heating fixing method has been widely used in a dry type electrostatic copying process. According to this method, an electrostatic latent image is formed on a photoconductive layer on the surface of a drum-shaped photosensitive body, and developed by a toner to form a toner image. The toner image is contact-heated by a heat roller and fixed on paper or the like. Though the contact-heating fixing method has the advantage of high thermal efficiency and rapid fixing, it has the drawback in which an offset phenomenon appears and the paper is wound on the heat roller.
In order to eliminate the above drawback, Published Unexamined Japanese Patent Application No. 1-309075 discloses a method for attaching wax to the surfaces of toners to give a separation property to the toners heated and fused by a heat roller.
The toners disclosed in the Japanese Publication are powders of adhesive resin containing a colorant and a charging control agent, and the wax is externally attached to the powders. Since, however, the wax is easily softened at high temperature and the softened wax has tackiness, if the toners are agitated at the time of development, wax particles are softened and attached to each other or to toner particles, resulting in cohesion of the toners. If the toners cohere with one another, the toner particles are increased and, even though the toners are charged by friction with carriers in twocomponent development, they cannot be sufficiently charged. For this reason, the toners peel off to contaminate the inside of a copying machine and a blur occurs to degrade image quality.
Since the surface energy of the toners disclosed in the above Japanese Publication is small, the wax is easy to separate from the surfaces of the toner particles when the toners are agitated in a developing unit. If the separated wax is charged and attached to a photosensitive drum, a black point appears on an image, thereby degrading image quality.
Using the toners to which the wax is attached as developers, the wax separated from the toner particles is attached to the photosensitive drum to form a film and thus to prevent a good image from being formed. It is thus necessary to remove the wax from the photosensitive drum by a cleaning roller. For this reason, the conventional method has a drawback in which the photosensitive drum is much worn to reduce its durability and thus to increase a workload of maintenance.